As a conventional exhaust gas sampling apparatus, as described in JPA 2000-221123, there is an apparatus of a constant volume dilution sampling type that samples the total amount of exhaust gas, and dilutes the exhaust gas with dilution air to provide a constant known flow rate.
Specifically, this exhaust gas sampling apparatus is provided with: an exhaust gas flow path through which the exhaust gas flows; a dilution gas flow path through which the dilution gas flows; a dilution tunnel that is connected with the flow paths to produce diluted exhaust gas that is a mixture of the exhaust gas and the dilution gas; a constant volume sampling device (CVS device) of a critical flow venturi type, which is provided on a downstream side of the dilution tunnel; and a sampling flow path for sampling part of the diluted exhaust gas flowing through the dilution tunnel to make the sampled diluted exhaust gas pass through a collection filter for PM collection.
Also, this exhaust gas sampling apparatus is, on an upstream side of the CVS device, provided with a heat exchanger for regulating temperature of the diluted exhaust gas to be constant. The heat exchanger regulates the temperature of the diluted exhaust gas flowing into the CVS device to be constant, and makes a diluted exhaust gas flow rate (flow rate of the diluted exhaust gas passing through a critical flow venturi) controlled by the CVS device constant. For this reason, by setting a flow rate (Qsamp) of the diluted exhaust gas to be sampled through the sampling flow path to a constant ratio (split flow ratio) with respect to the diluted exhaust gas flow rate (QCVS) controlled by the CVS device, a flow rate (Qfilter) of the diluted exhaust gas flowing into the collection filter is made constant.
On the other hand, there is an apparatus having a configuration in which in the above-described exhaust gas sampling apparatus, the heat exchanger is not provided on the upstream side of the CVS device. To set the split flow ratio, the exhaust gas sampling apparatus having such a configuration sets a diluted exhaust gas flow rate (QCVS) controlled by a CVS device to a flow rate (Qstandard_state) in a standard state (e.g., 20° C., 1 atm).
Meanwhile, in recent years, a flow velocity (Vfilter) of diluted exhaust gas passing through a collection filter has been limited. For example, in the exhaust gas testing regulations (CFR part 1065) in US, or the like, a limit where the flow velocity (Vfilter) of the diluted exhaust gas passing through the collection filter is 100 cm/s or less and close to the value is provided.
However, in the apparatus having the configuration in which the heat exchanger is not provided on the upstream side of the CVS device, temperature of diluted exhaust gas is increased during exhaust gas sampling, and along with the increase in temperature of the diluted exhaust gas, the flow rate of the diluted exhaust gas passing through the CVS device is reduced. As a result, using the setting flow rate (Qstandard_state) in the standard stat to make the split flow rate constant causes a flow rate (Qfilter) of the diluted exhaust gas passing through a collection filter to be also reduced along with the reduction in diluted exhaust gas flow rate during the exhaust gas sampling. For this reason, there occurs a problem that as the exhaust gas sampling proceeds, a diluted exhaust gas flow velocity (Vfilter) becomes separated from the regulated value.